1. Field of the Invention
This invention pertains to the field of chemical processing. More particularly, the present invention relates to a chemical process involving a processing step which is sensitive to the presence of at least one component contained within the stream to be processed and to an economical and efficient method of temporarily removing such deleterious component from the stream so as to have the deleterious component by-pass the step which is sensitive to this component.
2. Discussion of Related Art
There are many chemical processes in which there is at least one processing step which is sensitive to at least one component contained within the original feedstream to the process or to a component which is generated within the process upstream of the sensitive step. Generally, the presence of such a step will necessitate the removal of all or most of the deleterious component prior to its being introduced into the sensitive processing step.
These sensitive processing steps may include essentially all aspects of unit operations involved in chemical engineering practice. Thus, there are many chemical processes which cannot tolerate the presence of particular constituents which may be contained within the feedstream. For example, one such process involves the use of membranes for separating methane from natural gas where the presence of condensibles, such as pentane, hexane, or the like, would be detrimental to the membrane. So too, in those chemical reactions where a catalyst is employed, such catalyst is typically sensitive to various chemical constituents as well. Such sensitive catalysts include, for example, an iron oxide catalyst which is used for the formation of ammonia and which is particularly sensitive to carbon oxides. Without the removal of these deleterious components from the reaction zone, the catalyst will be poisoned, the reaction will not proceed, or proceed very poorly, or totally undesirable side reactions will take place.
Chemical reactions are not the only place in which the presence of certain components causes detrimental results. Thus, when using ion exchange resins, for example, it is frequently necessary to remove certain components from the stream to be processed prior to its being introduced into the ion exchanger. The presence of certain components within the feedstream could very well interfere with the ion exchange process or even destroy its utility completely. More specifically, in ion exchanging water to replace calcium ions with potassium ions, for example, the presence of sodium ions within the fluid stream would be detrimental to the ion exchange process requiring that the sodium ion be removed upstream of the process.
Even in certain distillation steps, particularly during azeotropic distillation, the presence of certain components within the fluid stream to be processed may be deleterious to the successful separation of the azeotropic solution. Again, this necessitates the removal of these constituents prior to the distillation step. The same holds true for still other unit operations, such as, irreversible adsorption when using zinc oxide, for example, and the like.
No matter which sensitive processing step is involved, it is readily apparent that steps must be and are taken to remove the deleterious components from the stream prior to such stream entering the sensitive step.
There may be situations, however, in which the deleterious component is not at all detrimental in the final product. Yet, because of the at least one sensitive step within the process, means must be taken to remove this component, usually at considerable outlay of capital cost for the necessary removal equipment and at increased overall operating expense.
Moreover, regardless of the means used to remove the deleterious component from the stream to be processed, it is still then necessary to deal with this removed component within the removal means. Thus, for example, when utilizing a solid adsorbent of the oxide type for the removal of the deleterious component, typically sulfide compounds, such an adsorbent is not readily regenerable. Hence, it is necessary to constantly replenish this adsorbent at considerable cost as well as deal with the ultimate disposal of the sulfide-laden oxide.
When fluid streams are utilized to remove the deleterious component, these streams too must then be regenerated for continued use requiring yet additional streams for such regeneration. Not only does this add to the cost of the overall process but there must also be a sufficient supply of such regenerating fluid as well. This is particularly true when using regenerable adsorbents such as molecular sieves. In order to desorb the deleterious component from the adsorbent, there must be a readily available supply of purge gas which must also be at the proper regenerating temperature. This is not always feasible at a particular plant site. Correspondingly, once the adsorbent has been regenerated with the purge gas, the purge gas, now laden with the deleterious component, must still be dealt with. Flaring of this purge gas is not always feasible or desirable.
One particularly prevalent deleterious component is sulfur and its compounds. Sulfur occurs in many industrial processes, and such sulfur, or sulfur containing compounds, must frequently be removed from process streams for various reasons. For example, if the process stream is to be burned as fuel, removal of sulfur from the stream may be necessary to prevent environmental pollution. Alternatively, if the process stream is to be treated with a catalyst, removal of the sulfur is often necessary to prevent poisoning of sulfur-sensitive catalysts.
A variety of methods are available to remove sulfur from a process stream. Most sulfur removal techniques involve the treatment of a gaseous stream. Such techniques include the use of alkaline reagents or an amine solution to remove sulfur or sulfur components from such gaseous streams. Alternatively, molecular sieves or other sorbents may be used such as a particulate oxide, hydrated oxide, or hydroxide of alumina, zinc, iron, nickel, cobalt, or the like, alone or in admixture with each other or with yet additional materials, e.g., alkali or alkaline earth metal oxides and the like. Reference is made to U.S. Pat. No. 3,492,038 which describes processes using such oxides. The use of molecular sieves as a sulfur removal adsorbent is discussed in, for example, U.S. Pat. Nos. 3,024,868, 4,358,297 and 4,533,529.
In general, however, solid adsorbents of the oxide type are not readily regenerable to their original form and must be discarded when they have become completely sulfided.
With molecular sieves, it is necessary to purge these sieves with a heated gas in order to desorb the sulfur components and regenerate them. The feasibility of such regeneration is in many instances limited by the quantity of gas available at a plant site for use as a hot purge gas.
One particular industrial process which requires the removal of both sulfur and nitrogen bearing compounds from the feed stream due to the use of sulfur-sensitive and nitrogen-sensitive materials within the process is the isomerization of a hydrocarbon feedstream containing at least five carbon atoms, particularly light straight run gasoline or light naphthas. Such a feed typically contains sulfur bearing compounds on the order of about 200 ppm of sulfur and nitrogen bearing compounds on the order of about 0-10 ppm. As used herein, the term "sulfur" is meant to include sulfur and sulfur bearing compounds and the term "nitrogen" is meant to similarly include nitrogen as well as nitrogen bearing compounds. Such levels of sulfur and/or nitrogen generally adversely affect the performance and life of the isomerization catalyst. Consequently, such a feed is conventionally treated by a hydrodesulfurization step to remove the sulfur and any nitrogen contained therein upstream of the isomerization step.
Such a hydrodesulfurization step generally involves a furnace heater to vaporize the feed stream, a hydrotreating reactor which catalytically converts the sulfur and any nitrogen present in the feed to hydrogen sulfide and ammonia, respectively, a condenser in which about 30 to 40% of the gaseous hydrogen sulfide and ammonia is condensed along with the feed with the remainder of the hydrogen sulfide and ammonia leaving as overhead, and a steam stripper column wherein the condensed hydrogen sulfide and ammonia contained within the feed is removed. In lieu of the steam stripper, a hydrogen sulfide and ammonia adsorption bed may also be used wherein the feed stream would have to be cooled to the proper temperature prior to entering the adsorber.
Regardless of whether a steam stripper or an adsorber is utilized to remove the hydrogen sulfide and/or ammonia, the hydrocarbon stream, now having essentially all of its sulfur and nitrogen content removed, must then be reheated to convert it to a vapor once again prior to being introduced to the isomerization reactor.
While such a hydrodesulfurization technique for sulfur and nitrogen removal is an effective means for dealing with the presence of sulfur and nitrogen, it is extremely costly. In fact, the conventional practice is to run the hydrodesulfurization (also known as hydrotreating) unit separately and independently from the isomerization unit which clearly adds to the complexity of the process and the overal costs. So too, the necessity of repeatedly having to heat and cool the feed stream so as to effect a phase change to accommodate different process steps also adversely affects the economics and efficiency of the overall process.
This is but one example in which a need clearly exists to be able to effectively remove at least one deleterious component from a feed stream in an industrial process which contains a step which is sensitive to this at least one component in an economical and efficient manner.